What is Inertial Confinement Fusion?

1. What is Inertial Confinement Fusion? Modeling 3. How do the spectra generated by the tracer layer in the ablator change over time? Radiation, Hydrodynamics, and Spectral Modeling of Indirect-Drive Fusion Experiments on the OMEGA Laser It takes three steps to create a simulated absorption spectrum for comparison with real data: Fusion is the joining together of hydrogen nuclei to form a more massive nucleus, releasing large amounts of energy in the process. SPECT3D is a spectral analysis program written by Joseph MacFarlane of Prism Computational Sciences that can take the hydro-dynamical output from Bucky and simulate a spectrum of the tracer in the ablator sample. 1. What is the time-dependent radiation spectrum incident on the ablator sample? VISRAD is a view-factor code written by Joseph MacFarlane of Prism Computational Sciences that allows us to model the radiation field inside the hohlraum and generate the spectrum incident on the ablator. Problem: the high temperatures and densities required to overcome the Coulomb barrier and join positively charged nuclei together result in high a pressure that tends to push fusion plasma apart. Simulated absorption spectrum of chlorine from a salt tracer in undoped plastic ablator. Spectrum was generated at 1000 ps using 841 atomic energy levels of chlorine. The fusing of Deuterium and Tritium (two isotopes of Hydrogen) into Helium. One solution: The inertial confinement of plasma. How do we learn things from looking at the chlorine absorption spectra? We take advantage of the affinity that X-rays have for inner shell electrons. 2p 2s energy When the radiation shock wave arrives at the NaCl tracer layer, it provides energy for electrons to make upward transitions resulting in K absorption lines. The appearance of these lines provides us with a local time- dependent diagnostic of the X-ray interaction with the ablator sample. Screen shot from VISRAD showing the lasers entering the hohlraum. A radiation spectrum incident on the ablator at 1000ps 1s This heating causes the outer layer of the plastic to ablate, meaning a rocket-like blow off ofmaterial. A spherical plastic fuel capsule filled with hydrogen is bombarded with energy. Conservation of momentum causes the fuel to move inward, reaching a core density 20 times that of lead. Note that the fuel is pushed inward, so its own inertia acts to impede its disassembly; hence the term inertial confinement fusion. The thermonuclear burning starts at the core and rapidly spreads throughout the fuel- this is referred to as ignition. hnx hnx Beryllium-like Helium-like Energy level diagram showing electrons making Kα transitions. A hydrodynamic simulation of the ablation and compression of a solid plastic sample by thermal X-rays. X-rays are incident form the left. David S. Conners, Katherine L. Penrose, David H. Cohen (Swarthmore College and Prism Computational Sciences) and Joseph J. MacFarlane (Prism Computational Sciences) Comparing the Model with Data undoped plastic 1.5% germanium dopant Energy can be indirectly delivered to the fuel capsule by firing lasers into an enclosure called a hohlraum that contains the capsule; The hohlraum converts the laser energy into thermal X-rays. Time = 600ps Time = 1000ps Time = 200ps The lasers deliver 7.5kilojoules of energy inside the hohlraum. Notice the “hotspots” where the lasers come in contact with the hohlraum. The ablator sample is the circular area on the far end of the hohlraum. In order to generate efficient fuel compression, it is important to control and diagnose the way the X-rays in the hohlraum interact with the fuel capsule. 2. What is the hydrodynamic response of the ablator sample to the hohlraum radiation field? Control: Adding dopants (like bromine or germanium) to the outer layers of the fuel capsule (the ablator) influences how the radiation field inside the hohlraum interacts with the ablator. Diagnose: The spectroscopy of NaCl tracer layers buried in the ablator can be used to measure time-dependent conditions in the ablator and thus to characterize the effects of doping on compression. BUCKY is a one dimensional hydrodynamics code that takes the radiation field spectrum output from VISRAD and models how the ablator sample’s position, density, and temperature change during the experiment. Changes over time in absorption spectra generated by tracer layers in the actual undoped and doped plastic ablators used in the April 2000 experiments. Note the delay in the appearance of Kαabsorption signals on the undoped and doped sides. Comparison of actual and simulated absorption spectra of salt tracer in undoped plastic ablator at 1000 ps. Future work includes comparison of data with model at all times for both doped and undoped ablator samples. Experimental Design Omega laser target chamber OMEGA Experiments April 2000 Conclusions • It is possible to measure backlit absorption spectra in a hohlraum radiation environment. • Doping ablator materials results in a later tracer turn-on time of chlorine Kα absorption lines, indicative of the slowing down of the radiation wave by the dopant . • Overall, for both the doped and undoped ablator samples, chlorine Kα absorption lines are present earlier than was predicted. Time dependent backlit Kα absorption spectroscopy of the NaCl tracer. Hohlraum radiation field parameters set in model to match DANTE measurements. DANTE is an absolutely calibrates ten channel X-ray detector. Acknowledgments We gratefully acknowledge the support of the Department of Energy through grant DE-FG03-98DP00250, the Delaware Space Grant Consortium. We would also like to thank Eric Jensen and all the Swarthmore students in the Physics & Astronomy department for their help and support over the last two years. Goal: To compare spectra of doped and undoped ablator samples. Bucky simulation of changes over time in the temperature of a doped plastic ablator exposed to thermal X-rays. Bucky simulation of changes over time in the density of a doped plastic ablator exposed to thermal X-rays. For more information or to download this poster, please visit http://astro.swarthmore.edu/~cohen/students.html